Various industrial elements benefit from the inclusion of a polycrystalline diamond compact (PDC), which may, for example, increase the thermal or abrasion resistance of the element or provide other benefits. The PDC portion of an element is typically formed by applying small diamond particles on the surface of a substrate, which typically forms the main body of the element. A catalyst may be applied with the diamond particles or, more typically, is present in the substrate and migrates out of the substrate during PDC formation. The diamond particles and substrate are subjected to extreme temperature and pressure, causing the diamonds to fuse together to form a generally continuous and physically solid matrix, the PDC. In this matrix there are substantial direct diamond-to-diamond bonds. After formation, the catalyst materials remain interspersed in spaces in the PDC matrix. The PDC is also bonded to the substrate in such a way that it will stay in place on the substrate when the PDC element is in use.
Typical catalysts include Group VIII metals, such as Cobalt (Co), Nickel (Ni), and Iron (Fe), and alloys thereof. These catalysts remain trapped within the diamond matrix when the PDC is formed. Although these catalysts can sometimes confer benefits to the PDC, such as improved mechanical strength, they can confer detrimental properties. For example, all of these metals have a much higher coefficient of expansion (tendency to expand when heated) than diamond such that, if the PDC element heats up during use, the metals may heat up and expand much faster than the diamond matrix, causing problems such as spalling, delamination, or conversion to graphite and ultimate failure of the PDC and the element as whole. These and other problems are described in UK Patent GB 2,422,623. Presence of the catalyst may also result in additional problems as well. As a result of detrimental effects of the catalyst, it is very often desirable to remove all or some of the catalyst material from the PDC.
Removal of the catalyst is typically accomplished using either an electrochemical or a chemical process. In chemical processes, referred to as leaching, strong acids, such as aqua regia (one part nitric acid, three parts hydrochloric acid), or caustic materials, such as NaOH, KOH, or halogen gasses are used to remove the catalyst from the diamond layer. Although some processes using these materials take place at room temperature, in other examples temperatures are elevated to as high as 800° C. Some processes also employ pressure vessels. Example processes are described in EP 1 960 158, GB 2,422,623, U.S. Pat. No. 6,749,033, U.S. Pat. No. 6,544,308, US 2007/0169419, U.S. Pat. No. 4,224,308, US 2005/0139397, US 2007/0181348, U.S. Pat. No. 7,608,333, and U.S. Pat. No. 4,288,248, although various other examples also exist.
Not only are such leaching processes generally extremely dangerous for a number of reasons, they also pose difficulties in protecting the portions of the element not to be leached. Control of the where the PDC element is leached may be important for at least three reasons. First, as noted above, the presence of the catalyst in the PDC does confer some advantages. As a result, it may be undesirable or not helpful to remove it from certain areas of the PDC, such as regions that are not exposed to such extreme heat or that benefit from the mechanical strength conferred by the catalyst. Second, more commonly, the substrate, which forms the bulk of the element, is typically made of a material whose resistance to harsh leaching conditions pales in comparison to that of the diamond matrix. Accordingly, exposure of the substrate to the leaching materials may cause serious damage to the substrate, often rendering the PDC element as a whole useless. Third, in some elements the presence of the catalyst in the PDC layer near the substrate is useful in maintaining the interface between the substrate and the PDC layer so that the PDC layer does not separate from the substrate during use of the element. It may therefore be important to protect the interface region from the leaching material.
Various systems for protecting non-leached portions of a PDC element include encasing the PDC element in a protective material, then removing it from the regions to be leached, coating the portion of the element to not be leached, and placing a physical cup around the protected portion then sealing the cup with an O-ring seal. Example protective systems include U.S. Pat. No. 7,757,792, U.S. Pat. No. 4,288,248, U.S. Pat. No. 7,608,333, and EP 1 960 158, although various other examples of proactive systems also exist. These systems tend to suffer from at least two detriments. First, many of them are not reusable and thus waste the protective materials. Due to the harsh nature of current leaching conditions, even reusable protective systems are often damaged or become unreliable after a small number of uses. Second, wicking of the leaching agent along the edges of current systems due to capillary action remains a problem. This can allow the leaching agent to reach areas of the element that should not be leached. At the very least this leads to unreliable leaching and in many instances, particularly if the leaching agent reaches the interface region or substrate, it can destroy the entire element.
Overall, current leaching technology would benefit from the development of less dangerous leaching agents and processes. It would also benefit from better protective systems, particularly systems that are more reliable and involve less waste.
As noted above, PDC elements and used leaching agents contain Group VIII metals. Although some Group VIII metals, such as Iron, are relatively safe and inexpensive, others, such as Cobalt, are highly toxic and costly. Accordingly, there is a need to recover and recycle Group VIII metals from PDC elements or other elements containing these metals, such as carbide elements, and from used leaching agents.